extensions.conf

[incoming_pstn]
exten => s,1,Dial(zap/1)

[outbound_pstn]
exten => _7.,1,Dial(zap/4/${EXTEN:1})



zapata.conf:

;;FXS modules
Group=1
callgroup=1
pickupgroup=1
signalling=fxo_ks
context=internal
channel=1

;;FXO Modules
Group=1
callgroup=1
pickupgroup=1
echocancel=yes
signalling=fxs_ks
context=incoming_pstn
channel=4